As more practical sealed batteries, lead acid batteries and nickel-cadmium batteries are in wide use at present The former is inexpensive, but in consideration of its weight efficiency/whkg.sup.-1, cycle life, etc. it is inconvenient to use the lead acid battery as a power source for portable apparatuses of the type for use over a long period of time. On the other hand, through the latter battery is relatively expensive, it has a capability of improving the defects of the former battery, and there is a remarkably increasing demand therefore of late, and in particular the products are of very wide application in fields requiring high reliabilities.
There is an earnest need for batteries of higher energy density employing such characteristic features and usable as a power source for portable appliances. General attention has been recently directed to the nickel-hydride batteries provided with a negative electrode using hydrogen absorbing alloys which are capable of absorbing and desorbing hydrogen as an active material electrochemically instead of cadmium electrode. This particular negative electrode is larger in energy density for a unit volume than that of the cadmium electrode. In this respect, while having the same volume as that of the cadmium batteries, a battery may be constructed with a negative electrode of smaller dimension than that of the cadmium batteries. That is to say, in such a structure, a greater quantity of positive electrode active materials can be used within the open space remaining inside the battery volume so that higher energy density can be realized.
As disclosed in U.S. Pat. No. 4214043, for example, the sealed type nickel-hydride battery is arranged such that its negative electrode is larger than the positive electrode in capacity and that there will always exist a precharged section (electrochemically active portion) at the negative electrode after complete discharge of the positive electrode.
The reason is that as compared with a nickel positive electrode, the negative electrode using hydrogen absorbing alloys is generally inferior in high rate discharge property, and thus a precharged section is disposed at the side of the negative electrode in advance to limit the battery capacity by the capacity of the positive electrode. This may prevent the negative electrode from reaching a fully discharge state or an over-discharge state even after the discharge of the battery, thereby to insure that any determination in cycle life due to a dissolution of the alloys of the negative electrode is avoided.
In this regard, the following methods have been proposed to construct a sealed battery provided with a precharged section at its negative electrode.
(1) As in the cadmium negative electrode, a negative electrode is charged electrochemically in an alkaline aqueous solution to set up a precharged section in the negative electrode, and then the negative electrode is wound in spiral together with a nickel positive electrode through a separator, followed by an infusion of an electrolyte into a case, and the case is sealed to form a battery.
(2) An alloy is ground through a hydrogen absorbing and desorbing operation, and some of the hydrogen used is absorbed into the alloy, i.e. a negative electrode is made of the alloy powders having precharged sections, and a sealed battery is produced in the same manner as described above (Japanese Pat. Laid-Open No. 62-154582).
(3) The positive and negative electrodes are wound in spiral through the separator, and these components are inserted into a cylindrical case with a subsequent infusion of an electrolyte. Then, a quantity of hydrogen gas necessary to establish a precharged section in the negative electrode is filled up inside the battery, and the battery is sealed, and a precharged section is established by a repeated charge and discharge so that a complete battery is formed (U.S. Pat. No. 4314043).
Said methods (1) and (2) require a complicated operation for a conventional negative electrode. Furthermore, in the methods (1) and (2), there is a risk that the active alloy with a lodged hydrogen might be burned by exposure to air prior to formation of a battery, and even if the alloy fails to catch fire, the hydrogen may be dispersed into the atmosphere, thereby to make it difficult to obtain a desired quantity of hydrogen in storage. The method (3) is a method for setting up a precharged section at the negative electrode with a hydrogen gas sealed inside the battery at the formation of the latter, in which the battery must be sealed in a hydrogen gas environment under high pressure, which creates a problem of the production procedures being very complicated.